This invention relates to an improved regulated alternating voltage source utilizing the effects obtained by ferroresonant transformers without their usual disadvantages. A general object hereof is to provide a low-cost, efficient, alternating current source that is tightly regulated and the output amplitude of which can be selectively adjusted. A related object is to provide such a power source that produces a substantially distortionless sine wave output and that responds rapidly in its regulating function to load changes and primary voltage changes.
The invention is herein illustratively described in its presently preferred embodiment; however, it will be recognized that certain modifications and changes with respect to details may be made without departing from the essential features thereof.
Ferroresonant voltage regulators are old in the art. Discussions of the operating principles of such systems occur, for example, in the following prior art reference materials:
Electronic Products, Dec. 1966, p. 58, et seq. PA1 Electronic Products, Jan. 1967, p. 74, et seq. PA1 Ieee transactions on Magnetics, Vol. Mag-6, No. 1, March 1970, p. 4, et seq. PA1 Ieee transactions on Magnetics, Sept. 1971, pp. 571, et seq. PA1 Ieee transactions on Magnetics, Vol. Mag-7, No. 1, March 1971 PA1 Ieee transactions on Magnetics, Sept. 1971, pp. 567, et seq. PA1 U.s. pat. No. 3,525,035, Aug. 18, 1970, R. J. Kakalec PA1 U.s. pat. No. 3,573,605, Apr. 6, 1971, H. P. Hart, et al PA1 U.s. pat. No. 3,573,606, Apr. 6, 1971, H. P. Hart, et al
The conventional open loop ferroresonant inverter comprises a ferroresonant transformer and a resonant circuit that, in conjunction with leakage reactance and a tuned filter, performs both the alternating voltage regulation and harmonic filtering functions. The circuit may also include a third transformer winding for cancellation of third harmonics in the output. Primary voltage may be either a sine wave, square wave or modified (quasi) square wave. While such inverters are comparatively simple, harmonic distortion is typically 5% or more, especially at high output voltage and light loading. Use of current feedback for regulation purposes increases output wave distortion, particularly when the primary voltage is a square wave or quasi-square wave. Since that portion of the transformer core within the resonating and output windings is saturated, filtering to reduce output wave distortion is difficult to achieve. This is especially so if operating efficiency is not to be materially reduced.
Furthermore, the degree of voltage regulation attained with such open loop ferroresonant inverters is insufficient for many applications even with constant load impedance. Regulation is considerably worse when variation in both input voltage and load are considered. In general transient performance is relatively poor since the open loop depends on high energy storage to perform its various functions. Fixed by the transformer characteristics and resonating capacitor value, current limiting is not adjustable in such inverter systems.
From stability considerations, the open loop ferroresonant transformer requires small air gaps in the magnetic shunts between primary and secondary. Thus, in order to assure operation above the knee of the B-H curve, circulating current in the resonant winding must be high, in turn requiring a large resonating capacitor and a bulky secondary winding.
In accordance with the present invention voltage regulation within .+-.1/2% under all practical operating conditions of line voltage variations and load variations is achieved whether these variations occur separately or simultaneously. Moreover, substantially distortionless sine wave output voltage, improved transient response, widely adjustable output voltage and higher efficiency are all attainable with apparatus requiring a ferroresonant capacitor less than half the size and only 70% of the winding copper and transformer core material required for the nearest comparable open loop type ferroresonant inverter. Further the improved inverter of this invention produces much less electromagnetic field interference in surrounding regions than conventional ferroresonant inverters. Utilizing a simple magnetic structure along with the other savings mentioned, the improved inverter also costs materially less to build.